The biggest weight savings here is the elimination of the hydro-mechanical constant speed drive either as a separate unit on older models or an IDG (Integrated Drive Generator) on the 767 and on.
Boeing used the same IDG on the 757, 767, and 747-400 (regardless of engine type). But the reliability of the IDG varied greatly between the installations - for example the MTBF on the 757/PW2000 was less than half that of the 757/RB211-535.
This was initially quite puzzling until someone thought to look at the typical and peak IDG oil temperatures between the various installations - turns out there was an almost linear inverse relationship between the oil temp and the IDG MTBF - the higher the oil temp, the worse the IDG reliability
For reasons that I don't recall, the PW2000 IDG had by far the highest operating oil temperatures.
It was bad enough that back around year 2000 we looked at what it would take to get the IDG oil temp down around that for the Rolls installation. Unfortunately all the identified fixes would be so expensive to retrofit that it was cheaper to simply keep replacing the IDG.
BTW radken, IIRC the 787 alternators are 150 KVA each, not 250...